The present invention concerns cellular mobile radio systems and more particularly managing radio resources in such systems and even more particularly managing radio resources in such systems during call handover.
FIG. 1 shows the general architecture of a cellular mobile radio system, for example the Global System for Mobile communications (GSM). A system of this kind essentially comprises:
a set of base transceiver stations (BTS) such as those designated BTS1, BTS2, BTS3 and BTS4, each base transceiver station being allocated to one cell, respectively designated C1, C2, C3 and C4, and the base transceiver stations communicating with mobile stations in those cells, such as stations MS1, MS2, MS3, MS4, in the example shown (other examples are possible, of course, with the same BTS allocated to more than one cell),
a set of base station controllers (BSC), such as those designated BSC1 and BSC2, each BSC controlling a subset of cells from the radio resource management point of view, base station controller BSC1 controlling cells C1 and C2 and base station controller BSC2 controlling cells C3 and C4 in the example shown,
a set of mobile switching centers (MSC), such as that designated MSC1, each MSC being connected to external networks, including the Public Switched Telephone Network (PSTN) or the Integrated Services Digital Network (ISDN) in particular, and controlling a subset of base station controllers from the call management point of view, mobile switching center MSC1 controlling base station controllers BSC1 and BSC2 in the example shown.
In a system of the above kind the network decides to hand over a call on the basis of a number of parameters including the results of measurements on radio signals for the current server cell and for neighboring cells of the current server cell.
The cell to which the call is to be handed over, called the new server cell, is chosen from a set of candidate cells selected by the network from the neighboring cells on the basis of the measurement results in the following manner.
Call handover to the best candidate cell is requested first. If handover proves possible, the best candidate cell constitutes the new server cell. If handover proves impossible, then handover to the next candidate cell is requested, in the order of preference of the candidate cells, and so on.
A candidate cell to which handover is requested is also known as a target cell.
A target cell to which a requested handover proves impossible is also called a rejected cell. Thus a target cell can be rejected, in particular because the mobile station does not succeed in setting up a link with the network in that cell, or because no radio resource is available in that cell because of a traffic overload, or because a checking or maintenance operation is in progress in that cell, the above list evidently not being limiting.
In the case of a target cell controlled by the same BSC as the current server cell, which corresponds in FIG. 1, for example, to a request to hand over a call from cell C1 to cell C2 or from cell C3 to cell C4 (handover of this kind is known as internal handover or intra-BSC handover), the BSC concerned is able itself to determine, internally, if handover to that target cell is possible or not, i.e. whether it is a rejected cell or not, because that BSC controls the radio resources for both cells.
In the case of a target cell controlled by a target BSC other than the current server BSC (referred to for simplicity hereinafter as the server BSC) controlling the current server cell (referred to for simplicity hereinafter as the server cell), which corresponds in FIG. 1, for example, to a request to hand over a call from cell C1 or C2 to cell C3 or C4 (handover of this kind is known as external handover or inter-BSC handover), the server BSC is not itself able to determine, internally, whether handover to that target cell is possible or not, i.e. whether or not it is a rejected cell, and the procedure used is then that outlined in FIG. 2 or 3, namely:
the server base station controller BSCs first sends the MSC a handover request message (xe2x80x9cHandover Requiredxe2x80x9d message HO REQ),
the MSC sends the target base station controller BSCc a message of the same type ( xe2x80x9cHandover Requestxe2x80x9d message HO REQ""),
the target BSC, after determining internally if handover to that target cell is possible or not, i.e. whether it is a rejected cell or not, sends the MSC either, in the case of a non-rejected cell (FIG. 2), a handover request acknowledgment message (HO REQ ACK) itself containing a handover command message, or, in the case of a rejected cell (FIG. 3), a message indicating that it is impossible to hand over to that target cell (xe2x80x9cHandover Failurexe2x80x9d message HO FAIL),
the MSC then sends the server BSC a message of the same kind, namely either a handover command message (HO COMM) or a message indicating that it is impossible to hand over to that target cell (xe2x80x9cHandover Required Rejectxe2x80x9d message HO REJ).
If a request for handover to a target cell receives a negative first response, it can be placed in a queue and repeated subsequently, within a predetermined time limit, this queuing technique being known per se. However, this technique may not be applied uniformly to all calls. For example, it may be decided that said predetermined time period can be relatively longer for an outgoing call, i.e. for a call sent by a mobile station, than for an incoming call, i.e. for a call received by a mobile station. The invention is not limited to an example of the above kind, however, and in particular covers the converse situation of deciding that said predetermined time period can be relatively longer for an incoming call than for an outgoing call, along with other examples of parameters beneficial for fixing said predetermined time period, or more generally for fixing the conditions under which a target cell is considered to be a rejected cell.
Here the general problem arises that in the case of external handover and when only the server BSC knows such parameters (which applies in particular to parameters consisting in the incoming or outgoing nature of the call), the target BSC may not apply said queuing technique optimally, or more generally any technique using such parameters to determine if a target cell is a rejected cell or not.
One aim of the present invention is to avoid this problem, i.e. to optimize the operation of the network in such cases, and more generally to improve co-operation between network entities in the event of call handover.
Accordingly the present invention consists in a method of co-operation between entities of a cellular mobile radio network for allocating radio resources in the event of call handover, wherein, in the event of a request to hand over from a server cell controlled by a server network entity to a target cell controlled by a target network entity separate from said server entity, in order to determine if handover to that target cell is possible, said target cell being referred to as a rejected cell if such handover proves impossible, said server entity informs said target entity of parameters used to fix the conditions under which said target cell is to be considered a rejected cell.
In accordance with another feature of the invention said parameters include parameters used to fix a predetermined time period during which a handover request of the above kind can be repeated if necessary.
In accordance with another feature of the invention said parameters include parameters relating to the incoming or outgoing nature of the call.
The present invention also consists in a cell control entity for cellular mobile radio networks, in particular a BSC for a GSM type network, for implementing a method of the above kind.